The present invention relates to a device for the spacing and holding in place of fuel rods in a fuel assembly of a pressurized water nuclear reactor.
In a fuel assembly, the fuel rods are arranged in bundle form as illustrated in FIG. 1. The fuel rods 2 are arranged in grids 4, 4a, and 4b. Two plates called the lower end plate 10 and upper end plate 12 are positioned on either side of the ends of the rods. A device called a spider 14 supports the absorbing rods 16, which are introduced to a greater or lesser extent into guide tubes 18, in order to control the operating characteristics of the reactor. These guide tubes 18 are located in the position of a few fuel rods.
The grids or spacing devices 4, 4a, 4b have to fulfill a number of functions:
the arrangement of the fuel rods in accordance with a precise, regular pattern, so that there is a sheet of water of given thickness between each rod to fulfill its two functions of neutron moderator and coolant; PA1 holding in place the fuel rods to prevent vibrations; PA1 axially supporting the fuel rods at their ends; PA1 an adequate mechanical strength and holding in place of the bundle in the case of transverse impacts between assemblies, e.g. resulting from an earthquake; PA1 a minimum hydraulic resistance to the outflow of the cooling liquid.
Conventionally, the assemblies of fuel rods have grids constituted by a system of recesses, where rigid supports such as bosses are provided, together with elastic supports such as springs exerting a force opposed to said bosses. Such an assembly is described in French Patent No. 2 168 059.
This type of assembly requires that the grids be sufficiently thick for the mechanical strength and rigidity to be adequate and this leads to a high hydraulic resistance. Moreover, the contacts between the grid and the fuel rods are relatively local. The guidance of the rods is consequently inadequate to prevent vibrations of said rods.